SIMULATION OF THE FERTILIZATION CA2-LAEVIS EGGS( WAVE IN XENOPUS)

In the preceding paper Fontanilla and Nuccitelli (Biophysical Journal 75:2079-2087(1998)) present detailed measurements of the shape and spe ed of the fertilization Ca2+ wave in Xenopus laevis eggs. In order to help interpret their results, we develop here a computational techniqu e based on the finite element method that allows us to carry out reali stic simulations of the fertilization wave. Our simulations support th e hypothesis that-the physiological state of the mature egg is bistabl e, i.e., that its cytoplasm can accommodate two alternative physiologi cal Ca2+ concentrations: a low concentration characteristic of the pre fertilization state and a greatly elevated concentration characteristi c of the state following the passage; of the wave. We explore this hyp othesis by assuming that the bistability is due to the release and re- uptake properties of the endoplasmic reticulum (ER) as determined by i nositol trisphosphate (IP3) receptor/Ca2+ channels and sarcoendoplasmi c reticulum calcium ATPase (SERCA) pumps. When combined with buffered diffusion of Ca2+ in the cytoplasm, our simulations show that inhomoge neities in the Ca2+ release properties near the plasma membrane are re quired tb explain the temporal and spatial dependences of the shape an d speed of these waves. Our results are consistent with an elevated IP 3 concentration near the plasma membrane in the unfertilized egg that is augmented significantly near the site of fertilization. These gradi ents are essential in determining the concave shape of the Ca2+ fertil ization wave front.